1. Field of the Invention
This invention relates to controlling a device mechanically positioned by one or more actuators while limiting the maximum rate and acceleration of the device and more particularly to a controller which modifies the position commands directed to a mechanically scanned antenna to limit the maximum rate and acceleration of the antenna with respect to its supporting structure.
2. Description of the Prior Art
It is necessary to control the rate and acceleration demands on a device positioned by actuators in response to position commands to avoid damage to the device and to improve the performance and settling time. For example, an antenna on an aircraft may be directed at a target while the airplane is undergoing maneuvers such as a roll. Position commands to keep the antenna directed at the target may exceed the rate and acceleration capabilities of the actuators or high torque motors of the antenna causing the antenna to be directed somewhere in space other than at the target or desired position. If excessive rate or acceleration is achieved in trying to redirect the antenna to a desired position, the antenna may inadvertently hit or be driven into gimbal stops which may damage the antenna. Also, if the rate and acceleration of the antenna is not properly controlled, antenna performance may be degraded by oscillations of the antenna about a desired pointing direction or by excessive time in reversing the antenna direction. Radar acquisition time of a target can be lengthened by these physical oscillations. Oscillations of the antenna have been damped out in some prior art airborne radar systems by using fluid drive actuators to position the antenna.
In antenna drive systems of the prior art, gimbal limits in azimuth and elevation were provided to ensure that the antenna was positioned within predetermined angle limits. For example, FIG. 8 of U.S. Pat. No. 3,924,235, which issued on Dec. 2, 1975 to Robert I. Heller et al. and assigned to the assignee herein shows gimbal limits for a reflector of a cassegrain antenna system driven by an antenna drive unit 18 in FIG. 1.
In addition to having boundary limits for the operation of the antenna, a rate limit in azimuth was imposed to further prevent the antenna from hitting gimbal stops during dynamic maneuvers of the aircraft.
In addition, a reduced gimbal limit boundary has been imposed in prior systems to serve as a buffer zone or safety margin to prevent the antenna from striking the gimbal stops and mechanically damaging the antenna. The reduced gimbal limit boundary provides an undesirable trade-off between scan coverage by the antenna and protection of the antenna from damaging itself.
One example of an aircraft maneuver which imposes high dynamic rates on the antenna drive unit is when an airplane may be moving in the direction of its longitudinal axis while undergoing a roll, for example, 180.degree. about the longitudinal axis. If the antenna is tracking a target at an angle off the longitudinal axis of the aircraft and close to the gimbal limits of the antenna prior to the roll maneuver, the antenna will move in an arc relative to its supporting members attached to the air frame as the airplane rolls. If the airplane rolls slowly, the antenna will track the target during the roll maneuver by the aircraft. As the pilot increases the rate of roll, the antenna will be driven faster and harder to maintain the antenna on the target. With increasing roll rate of the aircraft as controlled by the pilot, the antenna will fall behind the desired position of the target and will be driven harder to catch up to the target. The rate and acceleration of the antenna can exceed its mechanical control capabilities and the antenna may be driven into a gimbal stop due to its excessive velocity or due to the fact that the true position of the antenna is unknown since it already is lagging behind the desired position. When the pilot terminates a roll operation and the antenna catches up or reaches the desired position, the antenna velocity may carry the antenna beyond the desired point causing the antenna to oscillate about the point as it is driven in the direction of the desired point or target.
The antenna drive system may be an open loop system where the antenna is the recipient of a sequence of position commands without any feedback as to where the antenna is actually pointing prior to a position command. Alternatively, an antenna may have syncros positioned on the gimbal axes to determine accurately where the antenna is pointing. Normally in operation, the antenna pointing direction is determined in reference to target reflected signals which indicate the relationship of the antenna beam directed at the target and the line-of-sight of the target. In this situation, syncros would merely be helpful during dynamic maneuvers of the aircraft where the antenna is unable to be where it is commanded.
The path of a device such as a robot may be controlled by describing the path by a sequence of spaced-apart points to form line segments of equal length between the points. Precise control along the line segments is provided by interpolating additional points along the straight line segments for controlling the device along the straight line segments. U.S. Pat. No. 3,943,343, which issued on Mar. 9, 1976 to Akiyoshi Irie, entitled "Program Control System for Manipulator" describes such a system in FIGS. 3, 5A and 5B.
It is therefore desirable to provide a controller to control the path of an antenna or other device in an optimal manner by utilizing the sequence of position commands to derive direction and distance information.
It is further desirable to limit both the rate and acceleration demands on actuators such as the azimuth and elevation servos.
It is further desirable to smoothly slow a device with its oscillations damped before passing a desired point or a gimbal boundary by adjusting the rate and acceleration limits.
It is further desirable to control the path of a device without requiring the generation of trigometric functions to determine the path with respect to azimuth and elevation directions, for example.
It is further desirable to stop a device such as an antenna before reaching a selected point instead of passing the point to build up an error signal to stop the antenna.
It is further desirable under severe dynamic conditions such as when an airplane rolls, that the antenna is steered on an optimum recovery path away from gimbal boundaries and in the direction of the desired point.
It is further desirable that the controller would modify the antenna position commands only at times when acceptable rates and accelerations exceed desired levels.
It is further desirable to direct the path of a device such as an antenna along a shortened route within acceptable rates and accelerations to reach a desired position.